1. Field of the Invention
The present invention relates to a magnetic modulator coil driving circuit for supplying a current to a magnetic head for applying a magnetic field to a magneto-optic disk in recording data in the magneto-optic disk.
2. Description of the Prior Art
A method capable of overwrite employing magnetic modulation as shown in FIG. 3 has been proposed for recording data in a rewritable magneto-optic disk. This method irradiates the perpendicularly magnetizable film 10a of a magneto-optic disk 10 with a laser beam focused by the objective lens 11 of an optical pickup head to heat the perpendicularly magnetizable film 10a to a temperature not lower than the Curie point, and modulates a magnetic field created by a magnetic head 12 by applying recording signals to a magnetic modulator coil 12a by a driving circuit 13 to record the data in a magnetic pattern corresponding to the variation of the magnetic field in the perpendicularly magnetizable film 10a.
In recording data having a rectangular waveform as shown in FIG. 4A by magnetic modulation, a current Id supplied to the modulator coil 12a of the magnetic head 12 has an integral waveform having a time constant as shown in FIG. 4B. The intensity Hd of the magnetic field created by the magnetic modulator coil 12a has an integral waveform as shown in FIG. 4C substantially similar to that of the waveform of the current Id.
Referring to FIG. 5 showing a conventional magnetic modulator coil driving circuit, there are shown magnetic modulator coils L.sub.1 and L.sub.2, and switching elements Q.sub.1 and Q.sub.2, i.e., field-effect transistors. Modulating data D.sub.M applied to a terminal T.sub.2 is supplied to one input of each of two AND gates A.sub.1 and A.sub.2 whose outputs are connected to the gate electrodes of the switching elements Q.sub.1 and Q.sub.2, respectively. A control signal S.sub.WR is applied to a terminal T.sub.1 which is connected directly to the other input terminal of the AND gate A.sub.1 and, through an invertor I, to the other input terminal of the AND gate A.sub.2. The control signal S.sub.WR alternately opens and closes the AND gates A.sub.1 and A.sub.2 so that the modulating data D.sub.M is alternately supplied to the gates of the switching elements Q.sub.1 and Q.sub.2.
In this magnetic modulator coil driving circuit, a current flows through either the magnetic modulator coil L.sub.1 or L.sub.2 according to the modulating data to apply a magnetic field of either an N polarity or an S polarity to the magneto-optic disk 10. When either the switching element Q.sub.1 or Q.sub.2 is turned on, the magnitude of the current Id increases with time as shown in FIG. 4B. A level of the current Id is generated so that a magnetic field of a sufficiently high intensity is applied to the magneto-optic disk 10 even if the pulse width of the recording data is very small. However, if the pulse width of the recording data is comparatively large, an excessive current as indicated by the hatched area in FIG. 4B flows through the magnetic modulator coil L.sub.1 or L.sub.2 and, in the worst case, the magnetic modulator coil burns out due to overheating.
Since a control voltage that turns on the switching elements Q.sub.1 and Q.sub.2 in a saturation region has been used, it has been easy to release heat from the switching elements. However, it is difficult to cool the magnetic modulator coils because the magnetic modulator coils must have a lightweight construction to move the same together with the optical head.
Furthermore, temperature variation in the magnetic modulator coil entails the variation of the exciting current and an imperfect erasure of recorded data for overwriting.